Since you didn't really ask a question, I am assuming that you are wondering whether or not your mileage was "normal" for your rig. Given the numbers you did give, the mileage seems to be in the ballpark for your rig.

And yes, the stock 2.8 is a pig in terms of power. A good percentage of us that started with that engine are now running the GM 3.4, a HUGE upgrade.

You really can't judge mileage based on the miles per tank. You need to divide the miles driven by the number of gallons put in. The gauge is inaccurate enough that you really can't tell from just the fact that the tank needed filling whether you were taking on 18 or 22 gallons. Makes a huge difference in the calculated mileage.

Induced drag is also a big factor. Four big coolers are really going to increase the drag. Also, does your rig have a stock suspension and tires or is it lifted or running oversize tires. Either one can significantly reduce mileage.